The central objective of this Project is to use miniprotein mimetic together with the emerging structure of the gp120/CD4 complex to design antagonists of the interaction of human cells with HIV-1, the human immunodeficiency virus responsible for AIDS. T-cell docking and entry by HIV-1, a major route of cell infection in AIDS, is driven by specific recognition of the T-cell surface protein CD4 by the HIV envelope protein gp120. The crystallographic structure of CD4 is known, and that of its complex with gp120 is close at hand. Structural components in both protein partners have been identified which are proposed to play key roles in CD4-gp120 recognition. The advancing high resolution structural understanding of the protein participants in virus-cell recognition together with the advancing technology of mimetics design now make it possible to combine structure determination, modeling and miniprotein engineering to obtain an advanced mechanistic understanding of the structural basis of CD4-gp120 interaction and to design new antagonists for AIDS. The specific aims of this Project are: (1) obtain miniprotein constructions by transplanting CD4 and gp120 binding site components into conformationally constrained miniprotein presentation scaffolds by chemical and recombinant DNA methods and establish their CD4/gp120 antagonist activities; (2) construct phage displayed libraries of CD4- and gp120-mimetic miniproteins and use these to identify novel CD4 and gp120 binding antagonists as well as to examine the key structural elements needed for CD4/gp120 antagonist design; (3) determine the high resolution structures of key miniprotein constructs complexed with CD4 or gp120 and use the binding interface structures in these complexes to refine the topological map of key structural elements in CD4- gp120 recognition. Overall, this project will yield miniprotein mimetic constructions for both an immunorecognition receptor (CD4) and a retroviral envelope glycoprotein (gp120);and advanced mechanistic definition of key structural elements for CD4-gp120 interaction; and structural lead information for small molecule antagonist design. Long term, the mimetics strategies derived will be useful to determine mechanism of, and design antagonists for other protein-protein interactions important in AIDS, such as the gp41-gp120 and chemokine receptor interactions, that also are associated with human cell - HIV-1 docking and viral entry.